Form rolling is a manufacturing method that applies deformation processing on an outer peripheral surface of a rod-shaped material by driving a form rolling die in a state where die teeth that are formed on a form rolling die are pressed against the outer peripheral surface of the rod-shaped material. Generally, a rack die or a round die is applied as the form rolling die. The form rolling is widely applied for forming, for example, a helical gear, a screw, a down-sized worm, and a spline, as the manufacturing method that is environment-friendly, is highly productive, and is with low manufacturing costs.
The form rolling includes two types of manufacturing methods, which are, in-feed form rolling and incremental rolling. According to the in-feed form rolling, the die teeth of the form rolling die penetrate the outer peripheral surface of the rod-shaped material while gradually reducing a distance between the center of the rod-shaped material and the form rolling die to form a teeth portion on the outer peripheral surface of the rod-shaped material. Thus, in case of applying a round die for the in-feed form rolling, normally, die teeth having the same configurations are provided on the round die over an entire circumference of the round die. On the other hand, according to the incremental rolling, a teeth portion is formed on the outer peripheral surface of the rod-shaped material while maintaining a distance between the center of the rod-shaped material and the form rolling die to be constant. A form rolling die which is applied for the incremental form rolling is formed with die teeth having different configurations along an operational direction thereof. Thus, in case of applying a round die to the incremental form rolling, die teeth with different configurations are provided along a circumferential direction of the round die. Then, the outer peripheral surface of the rod-shaped material is applied with the form rolling to follow changes in the configuration of the die teeth.
JPS59-97731A (i.e., hereinafter referred to as Patent reference 1) discloses a form rolling apparatus which forms helical teeth on an outer peripheral surface of a rod-shaped material by in-feed form rolling. The form rolling apparatus disclosed in Patent reference 1 includes a support portion that supports a rod-shaped material to be axially rotatable, and a round die formed with die teeth on an outer periphery and positioned so that the die teeth face an outer peripheral surface of the rod-shaped material supported by the support portion. The following reference discloses an analysis of a process for the transition of contact states of a round die and a rod-shaped material from a rolling contact with friction state to a gear meshing contact state according to in-feed form rolling of the rod-shaped material and a method for reducing a work piece shift (axial motion) of the rod-shaped material in accordance with the transition of the contact states: Eiri NAGATA, Yoshitomo NAKAHARA, Morimasa NAKAMURA and Ichiro MORIWAKI. “Form Rolling of Helical Gear with Small Number of Teeth and Large Helix Angle (Reduction of Work Piece Shift)” Transactions of the Japan Society of Mechanical Engineers 79(798), 371-381 (hereinafter referred to as Non-patent reference 1). The following reference discloses numerical analysis of mechanism of the generation of form deviation during in-feed form rolling of the rod-shaped material: Eiri NAGATA, Tomokazu TACHIKAWA, Morimasa NAKAMURA and Ichiro MORIWAKI. “Form Rolling of Helical Gear with Small Number of Teeth and Large Helix Angle (Geometrical Discussion on Form Deviation Caused by Die Penetration)” Transactions of the Japan Society of Mechanical Engineers 79(807), 367-379 (hereinafter referred to as Non-patent reference 2).
Non-patent reference 2 confirms that, for example, a tooth profile deviation and/or undulation of a tooth trace is generated on a processed, or generated gear tooth (teeth) because of the die penetration of the round die to the rod-shaped material according to the in-feed form rolling using the round die. Non-patent reference 2 further discloses a method to cancel the undulation of the tooth trace by intentionally changing an axial phase of the round die and the rod-shaped material during the form rolling. According to this method, a certain effects for canceling the undulation of the tooth trace is attained, however, is not sufficient.
A need thus exists for a form rolling apparatus and form rolling method which is not susceptible to the drawback mentioned above.